1. Field of the Invention
The present invention relates to an apparatus for feeding sheets, separately in an image-forming device.
2. Description of the Related Art
A friction type pick-up member is adopted in the form of a roller or a belt made of a material having a high friction coefficient, such as a rubber, for a feed unit for feeding a sheet in an image-forming device such as an electronic photo copy machine, or a printer. The friction type pick-up member has a simple structure, but it needs to be pressed onto a sheet by a spring, etc., to obtain a large friction force. Furthermore, the friction type pick-up member cannot necessarily feed a sheet steadily as the friction coefficient of the material of the pick-up member initially having a high friction coefficient, such as a rubber, may change over a long period of time or in certain environments.
Instead of the friction type pick-up member, a suction type pick-up member which sucks in air over a sheet to pick up and convey the sheet by negative pressure, is also used. A sheet can be fed more steadily by the suction type pick-up member than by the friction type. However, a loud noise is generated when air is sucked in, and size of the apparatus is large so that the suction type feed system is unsuitable for use in an office and so on.
Furthermore, it often occurs that two or more sheets are fed at once in the friction type feed system so that a unit for separating the sheets individually must be provided to feed the sheets separately.
A friction pad made from a material having a high friction coefficient, such as a rubber, or a rolling member rotating in a direction so as to return a sheet to be fed, is pressed onto the pick-up member or a conveyance roller provided at the lower reaches of the sheet path, and a friction coefficient between the friction pad or the rolling member and a sheet is set to be smaller than a friction coefficient between the pick-up member or the conveyance roller and a sheet, and larger than a friction coefficient between sheets in order to feed sheets separately. This enables only one sheet on the side of the pick-up member or the conveyance roller to be fed when multiple sheets are supplied together, and enables a sheet to be fed against a friction force between the friction pad or the rolling member and the sheet, when only one sheet is sent by the pick-up member. Since this friction type separating system is simple, it has been widely adopted.
However, this system does not necessarily enable sheets to be separated when a vacuum state exists between sheets of a stack of sheets in a cassette for feeding, when sheets pull against or adhere to each other due to an electrostatic force, or when each sheet has a long nap on the surface thereof so that the nap of one sheet becomes entangled with that on the facing sheet.
Instead of the friction type separating system, a system for separating sheets individually having a conveyance roller that rotates in such a direction as to feed a sheet and a prevention roller rotatable in either direction, arranged so as to provide a predetermined torque in such a direction as to return a sheet facing another sheet so that a sheet path is formed between them, and positioned at the lower reaches of the sheet path is disclosed in Japanese Patent Application Laying Open (KOKAI) No. 56-7847, laid open on Jan. 27, 1981 (The corresponding U.S. Pat. No. 2459773). In this system, the prevention roller does not slide over the conveyance roller or a sheet when the prevention roller makes contact with the conveyance directly or when only one sheet is fed by the pick-up roller into a clearance between the conveyance roller and the prevention roller, so that abrasion of the roller, reduction of a friction coefficient, generation of fiber dust from a sheet, and reduction of a friction coefficient owing to the dust can be prevented. Sheets cannot be separated steadily when a sheet tightly contacts a facing sheet or the nap of a sheet becomes entangled with that of the facing sheet as mentioned above, as the difference in friction forces between a roller and a sheet and between sheets is utilized in this system also.
The present applicants et al. disclosed an apparatus for feeding sheets steadily one by one without a separating unit in an image-forming device, and also an apparatus for feeding sheets separately and steadily when multiple sheets are fed together by a pick-up member owing to a tight contact of the sheets, aimed at overcoming the above-mentioned drawback of the related art in Japanese Patent application No. 2-283707 filed Oct. 22, 1990 (The corresponding U.S. patent application Ser. No. 07/774349)
The first of the above-mentioned apparatus is shown in FIG. 12. The apparatus comprises an endless belt 2 for feeding a sheet. The belt 2 is made of a dielectric material, wound round rollers 5 and 6 and positioned over a stack 1 of sheets, the apparatus comprises an electrode member 3 for applying an alternating voltage from a power source 4 for an alternating current. The electrode member 3 is arranged to contact a surface of the endless belt 2, to thereby generate an alternating charged pattern on the surface of the endless belt 2. The stack 1 of sheets is carried on a movable plate 7 to be moved upward by a member 8 which press up against the plate 7, and a top surface of the stack 1 is held at a predetermined position. A sheet is fed along guide plates 10 and is conveyed by a pair of rollers 9.
An unequal electric field is caused near the surface of the endless belt 2 when an alternating charged pattern is generated on the surface of the endless belt 2 as mentioned above. A Maxwell's stress effect acts on a sheet 1a in the form of a dielectric contacting the endless-belt when a non-uniform electric field is caused near the surface of the endless belt, so that the sheet 1a is adhered onto the endless belt 2 by an electrostatic force. Accordingly, the sheet 1a is conveyed along a sheet path as a result of movement of the endless belt 2 around rollers 5 and 6. The attractive force owing to the charged pattern acts only upon the uppermost sheet 1a of the stack 1 and not upon the second sheet 1b so that feeding of multiple sheets can be prevented when a contacting pressure between the belt 2 and the top surface of the stack 1 is adjusted to be small.
The second of the above-mentioned apparatus is shown in FIG. 13 and 14. The apparatus comprises a friction type separating unit facing the endless belt 2 forming a sheet path between them, in which the unit is shown as a friction pad 110 of a rubber fixed on a base member 111 and always urged in an upward direction by a spring 112.
Sheets can be steadily separated by this unit as a sheet is strongly adhered by an electrostatic attractive force stronger than a friction force between sheets. However, the reliability of a separation of sheets is reduced when a friction force between the pad 110 and a sheet becomes nearly equal to a friction force between sheets due to a reduction in a friction coefficient between the pad 110 and a sheet over a period of time or owing to an adhesion of fiber du'st from a sheet and so on, as the prevention of feeding the lowest sheet of sheets that are being fed together, depends on a friction force.